The present disclosure relates to a gas turbine engine and, more particularly, to a thrust bearing system for a high pressure compressor section of the gas turbine engine that helps engine efficiency by decreasing bowed rotor shaft effects.
A gas turbine engine includes rotating spools or rotor shafts with blades that compress air needed for operation. One of the major goals in gas turbine engine fabrication is to optimize efficiency of the compressor and the turbine so that work is not lost. Tip leakage between the blades and the surround case is a significant source of engine inefficiency.
At shut-down of the gas turbine engine, the engine may cool unevenly. For example, the bottom part of the engine usually cools more quickly than the top portion of the engine as the heat within the engine migrates radially outward. This temperature differential in the gas turbine engine can cause the rotor shafts to distort and become eccentric relative to an engine center axis, taking on an effectively arcuate shape instead of being generally cylindrical. This phenomenon is also known as the “bowed rotor effect.” A “bowed” rotor shaft is problematic at engine start-up because the distortions in the rotor shaft may result in the blades rubbing into the surrounding case or rub strip thereby creating larger gaps between the blade tips and the surrounding case and increasing tip leakage and engine inefficiency. In some cases, the eccentric movement of the bowed rotor shaft can also create gaps between many of the components in the compressor and/or turbine sections of the gas turbine engine that can also cause air leakage and decrease engine efficiency. In addition to increasing tip leakage and engine inefficiency, a bowed rotor can also create noise and vibrations felt in the airplane at start-up.
One method previously used to deal with the bowed rotor shaft problem is to “jack” the gas turbine engine at shut-down. Jacking the gas turbine engine at shut-down is done by using a drive to slowly rotate the rotor shaft after shut-down until the engine is cool. However, this method is inefficient because jacking requires the addition of the drive and a power source for the drive to the engine, thereby adding to the weight and complexity of the gas turbine engine. Another method used to deal with the bowed rotor shaft problem is to “motor” the bowed rotor shaft at start-up. Motoring the bowed rotor shaft involves drive starting the gas turbine engine, i.e., rotating the bowed rotor shaft up to running speed prior to the addition of fuel to the gas turbine engine. However, motoring the engine can lengthen the start time of the gas turbine engine and can still cause vibrations during start-up. Some prior art engines include a roller bearing assembly in addition to the thrust bearing assembly to help dampen and reduce the affects the bowed rotor shaft at start-up. However, roller bearing assemblies only dampen radial play of the bowed rotor shaft while still allowing axial play and slop of the bowed rotor shaft and blades.